C++ Constructor for template of template

Question

So I have a template representing say a custom collection as follows:

template <typename T> class MyCollection {
    private:
        std::vector<T> v;

    public:
        MyCollection(int n) {
            v.resize(n);
        }
    etc
}

Now however, I want to instantiate a MyCollection of Mycollection objects, so I do this in my main program:

MyCollection<MyCollection<int>> mycoll= MyCollection<MyCollection<int>>(100);

This will indeed compile (on my MAC, using the following):

clang++ -std=c++11 -stdlib=libc++ test.cpp

The problem is that I get a linker error like this:

Undefined symbols for architecture x86_64:
"std::__1::__vector_base_common<(unsigned char)1>::__throw_length_error() const", referenced from:
  std::__1::vector<MyCollection<int>, std::__1::allocator<MyCollection<int> > >::allocate(unsigned long) in test-891fb4.o

What exactly is this and how do I fix it? It looks like it has something to do with a missing allocator for vector>? Why does this occur and how do I fix it?

Answer 1

Your constructor takes one argument. You are trying to construct the nested MyCollection without any arguments. That is not possible. You need to give a second constructor which takes an initializer for the inner collection.

#include <vector>

template <typename T>
class MyCollection
{
private:
    std::vector<T> v;

public:
    explicit MyCollection(int n) : v(n) {}
    explicit MyCollection(int n,T const& init) : v(n,init) {}
};

int main()
{
    MyCollection<MyCollection<int>> mycoll(100,MyCollection<int>(1));
}

Instead of the initializer you can also provide a default constructor.

#include <vector>

template <typename T>
class MyCollection
{
private:
    std::vector<T> v;

public:
    MyCollection() : v(0) {}
    explicit MyCollection(int n) : v(n) {}
};

int main()
{
    MyCollection<MyCollection<int>> mycoll(100);
}

Answer 2

Well well well, you're not going to believe this. It turns out the answer had nothing to do with the command line or libraries included. I had a *.h file of basic data types where I had this code:

#ifndef bool
#define bool unsigned char
#endif

Somehow, this messed with a prior definition of bool deep in one of the libraries, resulting in a link error that looked totally unrelated. Anyway, it works now.